1. Field of the Invention
The present invention relates generally to the field of holography. More particularly, the present invention relates to a shear-corrected digital hologram acquisition system suitable for use with “white light” (spectrally broadband) or laser illumination, or two-color illumination. For the two-color (more than two colors is also possible) implementation, the two colors may either both be broadband (low or very low coherence) illumination, or laser illumination. In one preferred implementation of the present invention, a laser is used for illumination, a mirror exclusive to the reference arm or corner mirror pair and an extended beam-combiner are used to create the phase-shift and shear-correction in the reference arm, and the hologram is recorded on a digital camera. The present invention thus relates to a digital hologram acquisition system.
2. Related Art
Prior methods of classical holography (1-4) and of digital hologram acquisition (5-9) have required both laser (coherent) illumination and that the reference and object (target) beams be combined at some angle (there is a shear between the two beams). Lasers have a number of problems, including high expense and generally requiring very extensive safety precautions, which makes them even more expensive. Additionally, since lasers have long coherence lengths (compared to broader band illumination sources), small reflections from optical surfaces will interfere with and make significant noise in the digital hologram. Previous methods have also required an angle (shear) between the two beams to create the spatially heterodyne fringe pattern that actually records the hologram. The shear is created by reflecting the reference beam from a mirror or beamsplitter so that it propagates at a different angle than the object (target) beam. In general digital holography systems require a small angle between the two beams; otherwise the fringes are too fine to be recorded by the digital camera or recorder being used. For common path systems, such as a Michelson geometry, or the last leg of a Mach-Zender geometry to the digital recorder, this means that the beams separate spatially from one another, and in fact makes it impossible to use a Michelson geometry for systems with high magnification—the reference beam becomes so separated due to the shear that it is either clipped by the optics, does not overlay the object beam, or both. Even with the shorter common path Mach-Zender layout, shear between the two beams often causes problems in achieving adequate overlay of the beams.
A system proposed by Thomas (10) teaches how to use a shearless system to solve the problem of non-overlapped beams of the sheared systems. The shearless system however requires generally expensive diffractive or holographic optical elements to eliminate the sheared beams problem.
FIG. 1 shows a prior art digital holography system with a Michelson geometry, where the shear between the two beams is indicated. Note that for this particular case, nominally a high-magnification case, the reference and object beams no longer have any overlap, as indicated, and therefore cannot form a hologram.